1. Field of the Invention
The present invention relates to a pulse power amplifier of the type which comprises a driver stage and a splitter/combiner stage following the driver stage for symmetrical power division.
2. Description of the Prior Art
A pulse power amplifier of the type generally set forth above is known from the Application Note AV-201, June 1978, of the Microwave Semiconductor Corporation.
High-frequency power transistors are employed for pulsed operation in pulse power amplifiers that generate peak pulse powers of 200W and above up into the GHz range. The obtainable pulse power of the individual transistors is limited and lies on the order of magnitude of 100W, dependent on the pulse format. For thermal reasons, the transistors thereby operate in the non-linear class C mode of operation and are only driven into the class A mode or class B mode during the gating process.
Higher output powers are achieved by parallel circuits having a plurality of transistors. The amplifier circuits thereby contain splitter/combiner stages as decoupling elements and in order to divide or, respectively, to combine the signals to be amplified or, respectively, the amplified signals. Dividers, in particular 3 dB power dividers or what are referred to as 3 dB hybrids are employed as splitter/combiner stages. Such "hybrid couplers" are known in the art, for example, from the catalog of the ANAREN Company. Power dividers such as 3 dB power dividers, are what are referred to as three-ports or four-ports that symmetrically divide power supplied at one port onto two other ports. In four-port power dividers, one port is always decoupled. In this connection, see R. K. Hoffmann, Integrierte Mikrowellenschaltungen, Springer Verlag, pp. 14-15. In the known amplifier circuits, this port is terminated with an absorber resistor.
The splitter/combiner stages are also employed given the division of the power supplied by the amplifier circuit onto two antennas.
In certain applications of pulse power amplifiers, it is not only a fixed, high output power that is required; rather, a maximum value and a minimum value of the output pulse power is required in view of low detectability and different distances to be bridged. The output pulse power can thereby differ by 10-30 dB.
It is conceivable to reduce the power output by the pulse power amplifier in that the drive power of one of the two output stage transistors connected in parallel to one another is disconnected on the basis of a switch-over device. What is referred to as a SPDT (single-pole, double-throw) switch can thereby be employed as a switch-over device. (In this connection, see Application of PIN Diodes, Application Note 922 of Hewlett Packard).
The switch losses must thereby be compensated by an increased driver power. Given reduced output power, the activated output stage transistor continues to work at full gain and power, this having a disadvantageous affect on the efficiency and on the reliability of the overall circuit.